Hard-to-kill algae and SWCG

[dawndenise]

I understand that when you shock a pool, it's best to go with bleach and turn off your SWCG. However, there've been a fair number of postings about a hard-to-kill algae that seems to require very, very high levels of chlorine to kill it, in excess of 25 and going much higher depending on your CYA levels.

If one needs that really, really high chlorine level, would it be better to leave the SWCG on at some medium level? Would that zap the nasties with a super, super high level of chlorine, at least within the cell? Would that help to get rid of that particular kind of algae faster? Or would the existing high shock levels of chlorine in the water make the SWCG less effective?

Just wondering because I may have that algae and my SWCG is currently off while I'm dosing the pool with bleach.

[cwstnsko]

When I shock my pool with liquid chlorine, I leave the SWCG on as normal. I don't turn it off, and I don't boost it, I just leave it set as normal. The way I think, it just causes the shock level to hang on a little longer and come down a little slower.

[chem geek]

Sandy,

When you add liquid chlorine or bleach to your pool, you are adding highly concentrated chlorine that will "super-shock" your pool in a local area as it mixes with the pool water. 6% bleach is over 60,000 ppm chlorine so even when it dilutes into 100 times the amount of pool water as the amount of chlorine you added, this is still 600 ppm. The SWCG has been measured by manufacturers to have around 80 ppm in the cell between the plates though I suspect that as one gets closer to the plate itself, the concentration is even higher. The only difference between adding the liquid chlorine or bleach manually vs. the SWCG is the rate at which this is done. The SWCG is more "continual" which probably makes it more efficient.

In both cases, however, only a local area gets super-chlorinated over a short period of time. So long as the algae is "mobile" in that it moves with the pool water, then circulation will eventually have that algae/water move through the central portion of the SWCG cell and get zapped. Manually adding chlorine would do the same thing, but you'd have to add it where the algae is located or would have to add a little chlorine almost continuously near a return to somewhat simulate what an SWCG was doing.

If the algae is stuck to a pool surface, then the SWCG system will do nothing to it. Manual dosing of liquid chlorine or bleach will kill the algae if applied where it is located. It appears that the hard-to-kill algae, which may be yellow/mustard algae, requires a high concentration of chlorine over an extended period of time, but due to CYA levels it turns out that the Free Chlorine level would only have to be 2 ppm to kill this algae if there were no CYA (at pH 7.5). It's the presence of CYA that reduces chlorine effectiveness and while regular green algae requires very little actual chlorine to die (perhaps 0.6 ppm FC if no CYA), this yellow/mustard algae requires more.

Of course, there's nothing like an experiment to verify a theory so I see nothing wrong with your running the SWCG while you are super-chlorinating. The efficiency of your SWCG will be a little lower (due to the higher chlorine concentration), but that's all. Of course, if the SWCG was able to zap this algae in the first place, then we wouldn't be seeing it.

In fact, has anyone seen the hard-to-kill mustard/yellow algae when using an SWCG system? This thread (http://www.poolforum.com/pf2/showthread.php?t=5612) has several users of this forum reporting this hard-to-kill algae, but I can't tell if any of them have SWCG systems.

Richard

[dawndenise]

Thanks for the replies. I'm convinced...back on goes the SWCG. Anything to help me get my blue pool back!!! :D